The present invention relates to a hydraulic pressure control device for an automotive automatic transmission.
In general, an automotive automatic transmission is composed of a planetary gear mechanism and a plurality of frictional engagement elements including clutches and brakes. In operation, the states of engagements of the frictional engagement elements are changed in various patterns so as to selectively connect and disconnect rotary elements of the planetary gear mechanism or to lock a rotary element or elements against rotation, thereby obtaining a speed stage which is optimum for the instant running condition of the automobile.
The control of the frictional engagement elements is usually performed by a hydraulic circuit having, through suitably setting a manual valve connected through a link to a manually operable shift lever disposed in the compartment of the automobile. Supply and discharge to actuators of the frictional engagement elements are performed by a plurality of speed-changing shift valves such as 1-2 shift valve, 2-3 shift valve, 3-4 shift valve and so forth. More specifically, a vehicle velocity signal and a throttle opening signal representative of the instant state of the automobile are input to an electronic control circuit which compares these data with a previously stored speed gear changing pattern, and solenoid valves incorporated in the above-mentioned hydraulic circuit are operated in accordance with the result of the comparison so as to control the speed gear changing shift valves thereby to set the automatic transmission in a speed stage which is optimum for the instant state of running. The pressure characteristics of the actuators of the frictional engagement elements are set by means of accumulators.
In recent years, there is an increasing demand for development of automatic transmissions intended for use on miniaturized automobiles. Such automatic transmissions are required to be small in size and to have 3 or 4 speed stages. This demand is met to a certain degree by designing the mechanical parts such as gear trains to have reduced sizes but is not fully met because of difficulty in the reduction of size of the hydraulic control unit, since the size of the hydraulic control unit is directly relates to the number of speed stage rather than to the size of the gear trains.
More specifically, in the conventional automotive automatic transmission, the hydraulic control unit incorporate various components such as shift valves which are operated in accordance with throttle opening and vehicle speed, orifice valves for controlling the control line pressure, accumulators for setting engaging pressure characteristics of the actuators for the frictional engagement elements, and so forth. An increased number of speed stages requires increments in the numbers of the shift valves and accumulators, resulting in an increase in the size of the hydraulic control unit. In addition, the hydraulic circuit is complicated and the weight, volume and the cost of the same are also increased, thus failing to meet the demand for reduction in the size and cost of automatic transmissions. The setting of the engaging pressure characteristic of each accumulator is conducted by adjusting an orifice and a spring. This makes it difficult to effect a delicate shock control in response to all of various speed gear changing conditions such as a throttle opening, vehicle velocity and oil temperature. In addition, such an accumulator is difficult to tune in accordance with a variety of types of automatic transmissions.
Another problem encountered with the known automatic transmissions is that a large time lag is involved in 2-3 and 3-4 upshifting operations due to the necessity for the operation of three frictional engagement elements, i.e., a CO clutch, a C1 clutch and a B1 clutch. In addition, since the clutch pressure cannot be controlled freely, a large shock is generated when a speed gear change is executed through switching of engagement between clutches which do not incorporate one-way clutch or between a clutch and a brake.
It is also to be noted that a considerably long time, e.g., several seconds or so, is required for the hydraulic oil to be drained at the time of shift from D, 2 or L range to P, R or N range. Therefore, if the shift lever is operated to effect a shift back to D, 2 or L range immediately after a shift to P,R or N range, the engaging pressure is established very quickly because of the residual hydraulic pressure,so that the clutch is abruptly switched into engaging state thereby to cause a shock.